Because of its clean burning qualities and convenience, natural gas has become widely used in recent years. Many sources of natural gas are located in remote areas, great distances from any commercial markets for the gas. Sometimes a pipeline is available for transporting produced natural gas to a commercial market. Although the transportation of gas by pipeline normally takes place over fairly lengthy distances, this would be no problem where only transportation over land is encountered. However, in many instances the natural gas is separated from a suitable market by expansive bodies of water. When pipeline transportation is not feasible, produced natural gas is often processed into liquefied natural gas (which is called "LNG") for transport to market. The liquefication plants are sometimes located at the source of the LNG, but the LNG plants are often located at ports from which the liquefied gas is shipped to foreign markets.
One of the distinguishing features of natural gas transportation systems is the large capital investment required. Pipelines, plants used to liquefy natural gas, and ships to carry the liquefied natural gas are all quite expensive. Pipeline materials and installation cost can be quite high and gas compressors and cooling systems arc required to move the gas through the pipeline. The liquefication plant is made up of several basic systems, including gas treatment to remove impurities, liquefication, refrigeration, power facilities, and storage and ship loading facilities. The design and operation of these systems can significantly increase the transportation cost of the natural gas. These systems can make transportation of the natural gas in some locations in the world economically prohibitive.
The development of natural gas fields in arctic regions, such as the North Slope gas and oil fields of the State of Alaska, present special challenges. The natural gas pipelines that are buried in frozen soil or permafrost must be taken into account. If such pipelines arc transmitting gas at temperatures above 0.degree. C. (32.degree. F.), the frozen ground in which the pipelines are buried will eventually thaw, and the resulting settlement or heaving action could possibly cause pipeline failure. Accordingly, preservation of the frozen soil or permafrost is a major concern to pipeline installers and operators, not only with a view to protecting the environment, but also with a view to minimizing damage and failure of the pipelines.
Various pipelines systems for conveying the natural gas in arctic environments have been suggested. U.S. Pat. No. 4,192,655 to von Linde discloses one example of a pipeline system for transporting natural gas over long distances in arctic regions by a pipeline to a liquefication plant at a port. The von Linde patent suggests using a pipeline having a number of sections in series with intermediate compressor stations. The pressure and temperature of the gas at the entry to each pipeline section is such that the drop in pressure of the gas in each section creates a drop in gas temperature and this low temperature gas is used to re-cool the gas heated by compression before it enters the next pipeline section. Von Linde suggests conveying the gas at an initial pressure of between 7,500 kPa (1,088 psia) and 15,000 kPa (2,175 psia) and at an initial temperature of below -10.degree. C. (14.degree. F.). The gas exiting the last pipeline section can be -45.2.degree. C. (-50.degree. F.) or lower. The liquefication plant, being located at the end of the last pipeline section, takes advantage of the low temperature in the liquefication process. From the liquefication plant the liquefied gas is pumped into tankers for transport to market.
Conventional gas liquefaction processes are required to produce a liquefied product that is below about -156.7.degree. C. (-250.degree. F.) for transportation via ships to the customer. As a result, more of the gas is consumed in the CO.sub.2 removal, gas liquefaction, and liquid regasification processes, thereby making less of the gas available to the consumer as product. In addition, gas transportation to the liquefaction facilities in conventional steel pipelines limits the practical (economical) operating pressure of conventional pipelines to pressures in the range of 6,895 to 15,860 kPa (1,000 to 2,300 psia), thereby requiring the use of gas recompressor stations along the pipeline route. The pipeline recompressors consume additional fuel and add heat of compression to the gas in the pipeline, so that the gas reaches the liquefaction plant at a warmer temperature than it would if pipeline recompression were not required.
The industry has a continuing need for an improved process for conveying natural gas which minimizes the amount of treating equipment required and the overall power consumption. By reducing the overall cost of conveying natural gas over long distances will add to the amount of gas available for use by consumers.